1. Technical Field of the Invention
The present invention relates to a semiconductor device for driving a current load device provided with a plurality of cells including a current load element and a current load device provided therewith, and particularly relates to a semiconductor device for driving a current load device for carrying out a gradation display by a current value to which a current load element is supplied and a current load device provided therewith.
2. Description of the Related Art
There has been developed a current load device provided with a plurality of cells, in the form of a matrix, including a current load element of which operation is decided by current supplied. Its application is, for example, a light emission display device in which a current load element is a luminous element, and an organic EL (Electro Luminescence) display device in which an organic EL element is used as a luminous element.
In the following, as a current load device, a light emission display device will be explained by way of an example. FIG. 1 shows the constitution of a matrix type light emission display device.
The display device comprises a horizontal driving circuit 200, a vertical scanning circuit 300 and a display portion 400. The gradation display is realized by adjusting current flowing in a luminous element within a 1-pixel display portion 100 of the display portion 400. In a luminous element whose brightness is decided by various current, current and brightness are in a proportional relation. By combination of the constitution of the 1-pixel display portion 100 and current or voltage applied from the horizontal driving circuit 200 and the vertical scanning circuit 300, the driving method of the light emission display device is classified into a simple matrix drive and an active matrix drive.
FIG. 2 is a circuit view showing the constitution of the 1-pixel display portion in case of the simple matrix drive. In the 1-pixel display portion 101 in case of the simple matrix drive, at each point of intersection between a control line 110 and a signal line 120, a luminous element 130 is connected between the control line 110 and the signal line 120. As shown in FIG. 1, the control line 110 is driven by the vertical driving circuit 300, and the signal line 120 is driven by the horizontal driving circuit 200. And, the control lines 110 are sequentially selected one by one by the vertical scanning circuit 300, and when current or voltage is output to the Lth signal line 120 from the horizontal driving circuit 200 during the scanning of the Kth control line 110, current flowing in the Kth line and the Lth column luminous element is decided, and the luminous element emits with intensity corresponding to the current. Thereafter, when the (K+1)th scanning is started, emitting of the Kth luminous element terminates.
FIG. 3 is a circuit view showing the constitution of the 1-pixel display portion in case of the active matrix drive. In the 1-pixel display portion 102 in case of the active matrix drive, at each point of intersection between the control line 110 and the signal line 120, a switch SW100 controlled by a potential of the control line 110 is connected to the signal line 110, and a gate of a TFT (Thin Film Transistor) T100 and one end of a capacity element C100 are connected to the other end of the switch SW100. A source of the TFT T100 and the other end of the capacity element C100 are grounded, and a luminous element 130 is connected between a drain of the TFT T100 and a signal line whose potential is VEL.
And, when the control lines 110 are sequentially selected one by one by the vertical scanning circuit SW300 and the Kth control line 110 is then selected, the switch 100 in the 1-pixel display portion 102 is turned on. At this time, the Lth output voltage of the horizontal driving circuit 200 is a gate voltage of the TFT T100, and when a gate voltage such that the TFT T100 is operated in a saturated area is applied, impedance of the TFT T100 is decided. As a result, current flowing in the luminous element 130 is decided, and the luminous element 130 emits with intensity corresponding to the current.
In the case of the active matrix drive, the 1-pixel display portion may sometimes take the other constitution. FIGS. 4A and 4B are respectively circuit views showing the other constitution of the 1-pixel display portion in the case of the active matrix drive. As shown in FIG. 4A, in a 1-pixel display portion 103 of the other constitution, a switch SW102 controlled by a potential of the control line 110 is connected to the signal line 110, and a gate and a drain of a P channel TFT T102 are connected to the other end of the switch SW102. A switch SW101 controlled by a potential of the control line 110 is connected to the gate and the drain, and a gate of the P channel TFT T101 and one end of a capacity element C100 are connected to the other end thereof. A constant potential VEL is supplied to sources of the TFT T101 and T102 and the other end of the capacity element C100. A luminous element 130 is connected between the drain of the TFT T101 and a ground potential GND. And, when the Kth control line 110 is selected by the vertical scanning circuit 300, and the switches SW101 and SW102 are turned on, a gate voltage of the TFT T102 is determined so as to cause the Lth output current of the horizontal driving circuit 200 to flow from the signal line 120. Since the TFT T102 and TFT T101 employ the current mirror constitution, where the current abilities of the TFT T102 and TFT T101 are equal to each other, the same current as the output current value of the horizontal driving circuit 200 flows to the luminous element 130 through the TFT T101, and the luminous element 130 emits with intensity according to the current value.
As shown in FIG. 4B, also in the case where N channel TFT T103 and T104 are used in place of the P channel TFT T101 and T102, the similar operation is carried out.
Comparing the simple matrix drive with the active matrix drive, in case of the active matrix drive, a voltage is stored in the capacity element even after next line is selected, and therefore, it is possible to continue to flow current. Accordingly, current allowed to flow to the luminous element is small as compared with the case of the simple matrix drive which merely emits momentarily.
As described above, even if the absolute value of current or voltage is different, where the gradation display is carried out, irrespective of the kinds of the driving methods of the simple matrix drive and the active matrix drive, the horizontal driving circuit 200 has a function to convert digital gradation data into current or voltage. In case of voltage output, since unevenness of threshold of a transistor and unevenness of voltage-current characteristics and current-brightness characteristics of the luminous element are present in a pixel circuit (1-pixel display portion), even if the same voltage is applied, there is a high possibility that brightness is uneven. On the other hand, in case of current output, being influenced merely by the unevenness of the current-brightness characteristics of the luminous element, unevenness of brightness is small, and high brightness can be displayed.
FIG. 5 is a block diagram showing one example of the constitution of a horizontal scanning circuit 200 for outputting current to a display portion 400. In this constitution, digital gradation data are developed to the number of output by a data logic portion 201, and afterwards, the digital gradation data are input into a digital voltage signal to analog current signal (digital-to-current) conversion portion 210 to thereby obtain a current output for the number of output.
FIG. 6 is a circuit view showing a first conventional example of a digital-to-current conversion portion for 1-output. Where gradation data are 3 bits (D0 to D2), switches SW110, SW111, and SW112 controlled thereby connected in common to an output end for outputting current I data. N channels TFT T110, T111, and T112 in which an input voltage VA is supplied to a gate are connected between the switches SW110, SW111, and SW112 and a ground wire at a ground potential VG. It is assumed that the current-brightness characteristics of the luminous element are in a proportional relation. Further, it is supposed that both the horizontal driving circuit 200 and the vertical driving circuit 300 are formed on a glass substrate, and all transistors are TFT. Even where gradation data are not less than 3 bits, the similar constitution is employed.
Further, in the first conventional example, it is designed so that with respect to the TFT T110, T111 and T112, the channel length (L) is constant, and the ratio of the channel width (W) is 1:2:4. Since TFT T110 to T112 are common such that the gate voltage is voltage VA and the source voltage is voltage VG, where TFT T110 to T112 are operated in a saturated area, the current ratio is 1:2:4. So, if a suitable input voltage VA is selected, switches SW110 to SW112 are turned on/off on the basis of gradation data D0 to D2 whereby with respect to the output current I data, current output of 8 gradations whose current ratio is 0 to 7 becomes enabled. Further, the absolute value of current can be regulated by changing the input voltage VA.
FIG. 7 is a circuit view showing a second conventional example of a digital-to-current conversion portion for 1-output. In the conventional second example, digital gradation data D0 to D2 are input into gates of N channels TFT T110 to T112. Drains of the TFT T110 to T112 are connected in common to output ends and a power supply voltage VD is supplied to sources thereof. The ratio of the channel width of the TFT T110 to T112 is set to 1:2:4 similarly to the first conventional example.
In the second conventional example as described above, a high level of digital gradation data input is set in advance to a suitable voltage, and a low level is made to be a level turned off by a thin film transistor, whereby current output of 8 gradations whose current ratio is 0 to 7 becomes enabled similarly to the first conventional example. Further, the absolute value of current can be regulated by changing a high level of digital gradation data input.
However, in a transistor, particularly in TFT, since unevenness of current abilities where the same gate voltage is applied between different TFTs is great, there poses a problem that it is difficult to issue a current output of high accuracy. In the conventional digital-to-current conversion portion, when there is a characteristic unevenness of TFT in substantially the whole width area of the display device, even the size of TFT is uniform and a voltage between the gate and the source is uniform, an uneven display occurs because the current value is different from that in other areas in the uneven portion. Further, current abilities become uneven even between TFTs as in a close area, and when such an unevenness becomes large, a display unevenness appears between close pixels, and when the characteristics of TFTs used for the same output become uneven, monotony of gradation is not satisfied.
Further, in the conventional digital-to-current conversion portion, particularly in the active matrix drive, there is a problem that where the output current value is low, it takes time for driving. This is because of the fact that when the active matrix drive by way of current drive is employed, driving completes at the time when the same current as the output current of the digital-to-current conversion portion as a driving circuit flows to the TFT in the pixel, but a wiring load, particularly a parasitic capacity is always present in the signal line 110 within the display portion 400, the luminous element also has a capacity value, and therefore it is necessary that the capacity loads are charged or discharged by output current which is constant current. That is, since the same current as output current of a digital-to-current conversion circuit which is a driving circuit flows to the TFT within the pixel first by charging or discharging the capacities to a certain voltage, it takes long time till then.